The present invention relates to an aqueous emulsion composition of a silyl group-containing copolymer. More particularly, the invention relates to an aqueous emulsion composition of a copolymer having a silyl group with good storage stability despite the high content of alkoxy or phenoxy groups bonded to the silicon atoms. The aqueous emulsion composition of the copolymer of the invention is useful as a vehicle or base ingredient of water-based coating compositions or adhesives.
It is a conventional technology that a coating composition or adhesive is imparted with improved adhesion to the substrate surface by introducing a silane coupling agent thereinto. A silane coupling agent can be introduced into such a composition either one of the following three methods including direct addition of the silane coupling agent to the composition, grafting of the silane coupling agent on to the base polymer in the composition and copolymerization of the silane coupling agent with another copolymerizable monomer. When the composition is a non-aqueous system having solubility in an organic solvent, any of the above mentioned three methods can be undertaken and the composition having a silane coupling agent introduced thereinto has no problems in respect of the adhesiveness and stability in the lapse of time.
In view of the problems of toxicity against human body and environmental pollution as well as the danger of fire caused by the organic solvent, organic solvent-based coating and adhesive compositions are in recent years under a trend of continuous shift toward water-based compositions. When a silane coupling agent is desired to be introduced into such a water-based composition, difficult problems are caused in respect of a decrease in the adhesiveness and loss of stability in the lapse of time because the hydrolyzable groups such as alkoxy groups in the silane coupling agent are readily hydrolyzed by water and the dehydration condensation takes place between the thus formed silanol groups eventually to cause gelation of the composition.
With an object to solve the above mentioned problems along with a possibility of improving adhesion as well as weatherability of a water-based coating or adhesive composition, a proposal has been made in Japanese Patent Kokai 2-308887 and elsewhere for an aqueous emulsion composition of a copolymer of a silane coupling agent and another copolymerizable monomer. As the silane coupling agent from which the above mentioned aqueous emulsion composition is prepared, two classes of organosilane compounds including vinyl-functional alkoxy silanes and methacryloxy-functional alkoxy silanes have been studies but these silane compounds have several defects. For example, vinyl-functional silane compounds have relatively low copolymerizability with other vinyl comonomers because the vinyl group therein is directly bonded to the silicon atom and under strong influence of the silicon atom so that a copolymer having a sufficiently high molecular weight can hardly be obtained resulting in a decrease in the adhesion when the vinyl-functional silane is used in a large amount while, when the amount thereof is small, the desired effect as a silane coupling agent can little be exhibited. When a copolymer is prepared from a methacryloxy-functional alkoxy silane, the resistance of the alkoxy groups against hydrolysis is low so that the aqueous emulsion composition has a problem in the storage stability.